1. Field of the Invention
The present invention relates to orthopaedic surgical implants and more particularly relates to an improved orthopaedic stem for use with an implant such as a hip implant or the like. Even more particularly the present invention relates to an improved flexible stem (or shaft) apparatus for use with an orthopaedic implant such as a hip implant wherein the stem includes a helical construct that will ultimately decrease the stiffness and thus improve the flexibility of the shaft portion of an orthopaedic implant.
2. General Background
Surgeons often implant surgical orthopaedic prosthetic devices that include a stem that occupies a portion of the patient's intramedullary canal. Examples include hip stems and tibial prosthetic stems. These prosthetic stems are well documented in the art as part of orthopaedic hip implants, knee implants and the like.
In the evolution of cementless femoral stem components, several problems have emerged. The main concerns are thigh pain and adaptive bone changes of proximal resorption and distal hypertrophy. Thigh pain has been attributed to some degree of instability between the femur and implant. Optimal stability can be achieved with modular components which permit ideal filling of the proximal and distal stem areas. A number of modular systems are now available through different manufacturers.
A growing body of literature supports the concept that flexibility of the stem tip is associated with a lower incidence of thigh pain. One study reports that once proximal bone ingrowth had occurred, severing the distal stem had no effect on the stability of the proximal implant and increased proximal strains. This is consistent with the concept that increasing flexibility at the stem tip combined with a rigid proximal metaphyseal portion will increase proximal load transfer.
One might infer that the stem is not necessary after bone ingrowth has occurred and an optimal stem should have a dissolvable distal stem. However, the distal stem is necessary for stability initially and probably to some extent long term. Applicant has observed late varus migration of certain prior art stems associated with poor filling of the stem tip on the lateral radiograph.
Stability of the stem within the patient's bone is achieved with optimal filling of the proximal and distal femur and may have little dependence on the elastic modulus of the stem material. Furthermore, optimal filling of the proximal and distal femur ensures that load is transferred to the proximal femur in both axial compression and torsion.
An ideal stem should thus consist of a flexible tip and rigid proximal section. This can be made with use of two different materials. A composite tip combined with a titanium proximal implant would achieve this. It also provides an opportunity for modularity, but presents a new interface since two materials contact one another. Designing a durable interface is essential to the feasibility of this implant.
Lack of adequate torsional stability in current stem designs is also a concern. Implants that increase torsional stability by obtaining distal fixation with extensively porous coated implants or distal flutes have been proposed. This is not ideal since torsional load transferred distally is unphysiologic and the proximal bone is stress shielded from torsional loads. A more physiologic stress transfer would ideally load the cortical bone of the calcar in combined axial compression and torsion.
Several patents have issued that relate generally to flexible femoral prostheses. U. K. Patent No. GB 2078523 B, entitled "Hip Joint Prosthesis", provides a hip joint prosthesis for replacement or restoration of the hip joint in a human body. The prosthesis includes a member that is adapted to be attached to the femur and a head portion adapted for cooperation with and movement relative to the acetabulum cavity of the pelvis or of a prosthesis defining such a cavity. The member has an elongated stem extending from the head portion and shaped for insertion into the medullary canal of the femur. The stem has a central axis and includes at least two rigid sections extending parallel to the axis and a resilient elastomeric material disposed between the rigid sections. The rigid sections are spaced laterally from the central axis and from each other.
U.K. Patent GB 2 239 398 A discloses a prosthetic implant having a varying modulus of elasticity. The implant has a shoulder portion and a stem portion comprising struts with a series of structures stacked thereon, the structures including both metal fiber structures and solid metallic structures which are arranged in such a manner that the flexural stiffness varies from one end of the stem to the other. Solid metal disks may alternate with fiber metal mesh disk regions. The relationship between the composite thickness of the mesh stacks and the thicknesses of solid disks determines the flexural stiffness in any particular region. Fiber metal segments of different thicknesses may alternate with solid metal plates. The implant may be particularly a hip prosthesis but may also be a knee, wrist, elbow, or shoulder prosthesis.
U.S. Pat. No. 4,292,695, entitled "Prosthesis Stem", provides an improved stem portion of a joint prosthesis for replacing a damaged or diseased skeletal joint, wherein a series of sections of elastomeric material attached to a rigid material are disposed one on top of the other in a staggered or offset configuration to form the stem. The offset is such that alternate sections of elastomer and rigid material contact selected locations of the bone within the medullary canal to firmly secure the prosthesis therewithin for resisting dislocation particularly during the early stages of postoperative rehabilitation.
U.S. Pat. No. 4,743,263, discloses an adaptable isoelastic hip endoprosthesis that comprises a joint piece connected to a shaft of the endoprosthesis which shaft is to be implanted in the femur, said shaft being composed of at least two spirally twisted elastic rods. Such rods can be of the same cross section, can have a variable cross section, and can have the same or different lengths.
U.S. Pat. No. 4,808,186, issued to T. S. Smith, provides a controlled stiffness elongated implant for use in the hip or other appropriate body joint. In the instance of the hip, a ball member fixed to the femur is rotatably engaged with a cup-shaped socket member fixed to the acetabulum of the pelvic bone. The ball member is mounted on one end of a femoral component which has an elongated stem receivable in the intramedullary canal of the femur. The stem has a longitudinal channel therein which lies generally in the coronal plane when the stem is in the implanted condition. The thickness of the stem laterally of the channel is variable between the proximal and distal ends so as to affect the moment of inertia at any given location along the length of said stem to thereby achieve stem flexibility which substantially correlates to the flexibility of the bone.
An orthopaedic prosthetic device possessing improved composite stem design is disclosed in U.S. Pat. No. 4,978,358. The surgical prosthetic device or implant of the '358 patent comprises a composite structure with an outer metallic component and a separate inner component comprised of the same or a different material. The outer component may be made of commercially pure titanium or a titanium alloy or of a cobalt-based alloy. The inner component may be made of a carbon composite material that may be reinforced or not reinforced with a polymeric material.
A load sharing femoral hip implant is disclosed in the Smith et al. U.S. Pat. No. 4,986,834. The '834 patent discloses a controlled stiffness elongated implant for use in the hip or other appropriate body joint. In the instance of the hip, a ball member fixed to the femur is rotatably engaged with a cup-shaped socket member fixed to the acetabulum of the pelvic bone. The ball member is mounted on one end of a femoral component which has an elongated stem receivable in the intramedullary canal of the femur. The stem has a generally longitudinally directed reduced mid-stem section. The dimension of the reduced mid-stem section is uniform or variable between the proximal and distal ends so as to affect the mass moment of inertia at any given location along the length of said stem to thereby achieve an optimal stem flexibility.
U.S. Pat. No. 5,030,234, entitled "Prosthetic Device With Modular Stem", provides a modular stem type prosthesis which includes a stem and an extension which are connected to one another with a slip fit interconnection that minimizes surface tensile forces in regions of the prosthesis adjacent the interface between the stem and the extension. Engagement between the stem and the extension is provided by deflectable end portions of one component of the prosthesis which are engaged in a mating structures may define an interfitting ridge and groove. Micromotion between the respective parts may be prevented by a screw which may be tapered to achieve a lock fit. The extension may be of any selected length and any selected diameter in accordance with the needs of the patient.